System and methods for conducting an interactive dialog via a speech-based user interface

ABSTRACT

A system and methods are provided for conducting an interactive dialog via a speech-based user interface. The system includes an auditory prompt module for generating a sequence of goal-directed auditory prompts based upon a pre-determined user-oriented task, and an error recovery module for responding to a communication error by initiating an error recovery based upon a user response to the sequence of goal-directed auditory prompts. The error recovery includes providing at least one error-recovery prompt selected from a hierarchy of error-recovery prompts, the selection based upon a hierarchy of communication error types.

BACKGROUND

1. Field of the Invention

The present invention is related to the field of speech communication,and, more particularly, to speech-based user interfaces.

2. Description of the Related Art

A user interface is the component that allows a user to input into acomputer system the various instructions and/or data used by thecomputer system in carrying out any of a multitude of processing tasks.Various types of user interfaces presently exist, most being implementedthrough a combination of hardware or software. One of the most promisinguser interface types, both in terms of future uses and furtherdevelopment, is the speech-based user interface.

The speech-based user interface has dramatically expanded the range ofuses to which computers can be put. The speech-based user interface can,in many circumstances, obviate any need for text-based input via akeyboard or graphical input via a graphical user interface (GUI). Thisenables a user to access a computer system without using a computerterminal, an occurrence common when interfacing with embedded devices,small mobile devices, and telephony-based voice-response systems.Consequently, the speech-based user interface has extended the reach ofvarious types of computer systems by making them usable simply on thebasis of ordinary spoken language. Indeed, as a result of thespeech-based user interface, there is an expanding array of computersystems for conducting banking transactions, making airlinereservations, and carrying out a host of other functions merely on thebasis spoken commands.

Speech-based user interfaces typically rely on automated speechrecognition for converting speech into machine-readable instructions anddata. In general, speech recognition involves the transformation ofacoustic signals into electronic signals that, in turn, are digitizedfor electronic processing by the speech recognition device. Regardlessof the sophistication of the underlying technology, however, there isinevitably some risk inherent in any user-machine dialog via aspeech-based user interface that a communication error will occur, justas there is in the case of human-to-human communications.

Accordingly, conventional speech-based user interfaces typically rely ontechniques such as help messages and supplemental information provided,for example, by a text-to-speech (TTS) processor to guide a user throughan automated, speech-only exchange or dialog with a computer system orspeech recognition device. Often times, though, users of speech-basedinterfaces find these help messages and informational guides repetitiveand unhelpful. Many speech-based user interfaces operate as though onlytwo types of communication errors occur: either the user is unaware ofthe need for a user utterance at some point during an interactivedialog, which results in a time-out error, or secondly the userutterance given is unrecognizable to the user interface because it isnot part of the user interface grammar.

In many contexts, however, communication errors are more complex, andthe conventional assumption about the nature of the errors is thusoverly simplistic. Conventional speech-based user interfaces thus tendnot to address speech recognition communication errors in a mannercomparable to the way such errors are recognized and handled in ordinaryhuman-to-human conversations. Accordingly, the processes employed forerror recovery with such speech-based user interfaces do not reflect thenatural responses that would follow from a human-to-humanmiscommunication or communication error. This, in turn, can makeuser-machine dialogs via a speech-based user interface less pleasant andmore difficult as compared with human-to-human communications.

Moreover, conventional error recovery processes also tend to lack thecomplexity needed to deductively determine the nature of a communicationerror in a dialog via a speech-based user interface. It follows, thatthese processes also typically fail to provide a targeted response to acommunication error.

SUMMARY OF THE INVENTION

The present invention provides a system and methods for conducting auser dialog via a speech-based user interface. A system according to oneembodiment of the present invention can include an auditory promptmodule that generates a sequence of goal-directed auditory prompts. Thesequence of goal-directed auditory prompts can be based upon apre-determined user-oriented task. The system can further include anerror recovery module responsive to a communication error. The errorrecovery module can initiate an error recovery based upon a userresponse to the sequence of goal-directed auditory prompts. In carryingout the error recovery, the system can include providing at least oneerror-recovery prompt sequentially selected from a hierarchy oferror-recovery prompts. The sequential selection of an error-recoveryprompt can be based upon a hierarchy of communication error types.

According to another embodiment, the error recovery further includesdetermining a communication error type. A determination can be basedupon one or more responses of a user response to one of the sequence ofgoal-directed auditory prompts. The determination of communication errortype, moreover, can be based upon a sequential analysis of userresponses to the sequence of goal-directed auditory prompts.

According to still another embodiment, a method for conducting a userdialog via a speech-based user interface can include providing asequence of goal-directed auditory prompts, the sequence ofgoal-directed auditory prompts based upon a pre-determined user-orientedtask. The method also can include responding to the communication error,if one occurs, by initiating an error recovery based upon a userresponse to the sequence of goal-directed auditory prompts. The errorrecovery can include providing at least one error-recovery promptselected from a hierarchy of error-recovery prompts, the selection beingbased upon a hierarchy of communication error types.

Yet another embodiment of the present invention is an apparatus forconducting a user dialog via a speech-based user interface. Theapparatus can include a computer-readable storage medium. Thecomputer-readable medium can store computer instructions for providing asequence of goal-directed auditory prompts, each based upon apre-determined user-oriented task, and for responding to a communicationerror by initiating an error recovery if a communication error occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings, embodiments which are presentlypreferred, it being understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

FIG. 1 is a schematic diagram of a computing device that includes asystem for conducting a user dialog via a speech-based user interfaceaccording to one embodiment of the present invention.

FIG. 2 is a flowchart illustrating a method for conducting a user dialogvia a speech-based user interface according to one embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic diagram of a computing device 100 having aspeech-based user interface 102. As illustrated, the speech-based userinterface 102 is communicatively connected to a system 104 forconducting dialogs via the user interface according to one embodiment ofthe present invention. The dialogs conducted by the system 104 serve asa mechanism by which a user enters instructions and/or data into thecomputing device 100. The computing device 100 can be any device forprocessing instructions and data in the form of electrical signals, andhaving a speech-recognition capability such that the instructions anddata can be supplied by voice input. Accordingly, the computing device100 can be, for example, a general-purpose computer such as a main-frameor personal computer (PC) that includes dedicated circuitry and/orsoftware for implementing speech recognition. Alternately, the computingdevice 100 can be a special-purpose computer such as a voice server.Still other examples of the computing device 100 include a mobile phone,a personal digital assistant (PDA), and any of a host of embeddeddevices familiar to those of ordinary skill in the art.

In general, the computing device 100 can be based on a standardarchitecture and can include a processor (not shown) for executinginstructions and memory (also not shown) connected to the processor forstoring instructions and data entered by the user via the speech-baseduser interface 102. The speech-based user interface 102 isillustratively implemented as a software program that can be configuredto be stored in the memory and executed by the processor of thecomputing device 100. Similarly, the speech recognition module 108 isillustratively implemented by software-configured to be run on thecomputing device 100. The speech recognition module illustrativelyoperates cooperatively with the speech-based user interface 102 intranslating the user-supplied voice signals into the digitizedinstructions and data that can be stored and processed by the computingdevice 100. Although illustratively implemented as software-basedinstructions, either or both of the speech-based user interface 102and/or the speech recognition module 108 can be implemented usingdedicated hardwire circuitry or a combination of dedicated hardwirecircuitry and software.

Instructions and data are illustratively entered into the computingdevice 100 by the user speaking into a microphone 106 that is connectedto the computing device 100 for receiving user-supplied acousticsignals. As will be readily understood by one of ordinary skill in theart, these same acoustic signals are illustratively transformed intoelectrical signals and digitized for processing by the computing device100 using the speech recognition module 108 incorporated within thecomputing device as illustrated.

The instructions and/or data entered into the computing device 100 bythe user can be used to control various processing activities carriedout by the computing device. Thus the instructions and data can be asvaried as the many applications for which the computing device 100 canbe used. For example, if the computing device 100 is a voice server, auser can carry out various functions or transactions over a conventionaltelephone communicatively linked to the speech-based user interface 102of the computing device. Such transactions include conducting financialtransactions with a designated bank or making reservations with adesignated airline by way of spoken instructions and/or informationtransferred over the telephone into the voice server via thespeech-based user interface 102. Another example would be if thecomputing device 100 serves as a interactive controller for an auxiliarydevice such as a machine or a multi-task processing plant. In suchevent, control is effected by the user issuing spoken commands to thecontroller via the speech-based user interface 102. Various other typesof transactions, as well as more general applications, can similarly beperformed depending on the particular nature of the dialog conducted bythe system 104, as explained in more detail below.

Though not specifically shown, it will be readily understood by one ofordinary skill in the art that the speech recognition module 108 caninclude algorithmic modules based on one or more models such as anacoustic model, a lexical model, and a language model. Illustratively,the speech recognition module 108 generates a spectral representationderived from the digitized, time-varying signal by sampling the signal,typically at a fixed rate such as once per every 10 or 20 milliseconds.Through a process of feature detection, segmentation and identification,and matching, the speech recognition module 108 causes the computingdevice 100 to generate computer-readable instructions and data elementsfrom the spectral representation of the user-supplied voice input.

For each task to be performed by the computing device 100, there can bea corresponding dialog that comprises the particular speech-basedinstructions and/or information that is entered by the user via thespeech-based user interface 102 to effect the specific task. Theparticular dialog is conducted or controlled by the system 104. Thesystem 104 illustratively includes an auditory prompt module 110 thatgenerates a sequence of one or more auditory prompts. Each of theseauditory prompts is a goal-directed prompt in the sense that eachsequence of one or more prompts corresponds to the particular dialog forcompleting the specific user task. The dialog can vary widely and can bedesigned to perform any number of tasks. For example, as alluded toabove, the dialog may be designed to allow the user to conduct anautomated financial transaction, make an airline reservation, control amachine or plant, or accomplish various other tasks based upon verbalinstructions and data entered via the user interface 102.

A simple example of a dialog that could be conducted by the system 104involves an automated directory assistance application. According tothis example, the computing device 100 operates as a voice serverconnected, for example, to a telephone network over which callers areable to access the voice server. Through a telephone connection to thevoice server, a caller supplies speech input to the voice server via thespeech-based user interface 102, and prompts are conveyed to the callervia the text-to-speech module 112. The goal to which the prompts aredirected is the caller's ascertaining the number of the person thecaller is trying to reach. The following illustrates a dialog that couldbe conducted by the system 104 for this scenario provided that nocommunication errors occur:

-   -   (System): “Welcome to directory assistance. What is the name of        the person you wish to contact?”    -   (Caller): “Bob Jones.”    -   (System): “Thank you. Mr. Jones' office phone number is        (555)-852-1212. Do you wish to be connected directly to Mr.        Jones?    -   (Caller): “Yes.”    -   (System): “Your call is being connected now. Please hold.”

Thus, in general, each of the one or more goal-directed prompts of aparticular sequence corresponds to a particular user-oriented task andis intended to illicit a certain type of response from the user.Illustratively, the goal-directed prompts are implemented as verbalprompts generated by a text-to-speech module 112 configured forexecution by the computing device 100 and conveyed to the user via aspeaker 114 connected to the computing device. Other mechanisms also canbe employed for conveying goal-directed prompts to a user, such as, forexample, via visual prompts conveyed to the user via a visual interface(not shown) connected to the computing device 100. Alternately, theprompts can be audible prompts conveyed to the user via a hardwired orwireless connection. These other mechanisms can be employed in lieu ofor in conjunction with the verbal prompts conveyed directly to the uservia the speaker 114.

Illustratively, the system 104 further includes an error recovery module116 for responding to a communication error. Communication errors canarise in distinct ways. The user's speech may be too loud or too soft,for example. The user's speech, alternatively, may be unrecognizable bythe speech recognition module 108. Even if the user's speech comprisesrecognizable sounds uttered at an appropriate volume, the specific wordor words may have meanings that do not communicate an appropriateresponse in the context of a particular prompt. Apart from therecognizability and meaning of the individual words, their syntax in thecontext of a particular dialog may not make sense. Accordingly, acommunication error or interruption in a particular dialog can stem froman acoustical error, a lexical error, a syntactical error, or aconceptual error, for example. The error recovery module 116 handlessuch communication errors in the manner described below so that theparticular dialog can be completed.

In response to a communication error, the error recovery module 116initiates an error recovery. The error recovery is carried out by theerror recovery module 116 providing at least one error-recovery prompt.Each error recovery prompt is selected from a hierarchy oferror-recovery prompts. The particular error-recovery prompt supplied bythe error recovery module is selected based upon a correspondinghierarchy of communication error types. Illustratively, the hierarchy ofcommunication error types includes acoustical errors, lexical errors,syntactical errors, and conceptual errors.

The error recovery can be effected by the error recovery module 116determining whether an acoustical error has occurred based upon theauditory level of the user response to at least one of the goal-directedauditory prompts provided by the auditory prompt module 110.Alternatively, the error recovery module 116 can carry out the errorrecovery by determining whether an acoustical error has occurred basedupon an indeterminate response, in which event the goal-directed promptmay be repeated, with or with emphasis on main content words.

In the event that an acoustical error is determined by the errorrecovery module 116 to have occurred, an acoustical error prompt isprovided in response thereto by the error recovery module. Theacoustical error prompt, for example, can prompt the user to repeat aresponse at a different volume if the initial user response was eithertoo loud or too soft. Alternately, if the acoustical error is determinedon the basis of the indeterminate grammar, the error recovery module 116can provide the user with a repetition of the goal-directed prompt towhich the user was responding. The repetition optionally can emphasizecertain main-content words to make the goal-directed prompt moreunderstandable by the user.

For example, in the context of the earlier scenario concerning automateddirectory assistance, a communication error and error-recovery may occurin the dialog as follows:

-   -   (System): “Welcome to directory assistance. What is the name of        the person you wish to contact?”    -   (Caller): INDETERMINATE RESPONSE (e.g., “huh?”)    -   (System): “Please provide a name for the person you wish to        contact.”

Additionally, the error recovery module 116 can carry out the errorrecovery by determining whether a lexical error has occurred based upona search for a match between a user response and a list ofpre-designated acceptable responses, where both the user response andthe list of pre-designated acceptable responses correspond to the samegoal-directed auditory prompt. If there is no match, then the errorrecovery module determines that a lexical error has occurred. Errorrecovery is accomplished by the error recovery module 116 providing alexical error prompt in response to the lexical error. According to oneembodiment, the lexical error prompt can provide the user with a list ofappropriate responses for the particular goal-directed prompt to whichthe user was responding when the lexical error occurred.

The error recovery module 116 also can effect the error recovery bydetermining whether a syntactical error has occurred based upon aparts-of-speech evaluation of the user response and a search for a matchbetween parts of speech identified by the parts-of-speech evaluation andparts of speech contained in a set of expected parts of speech. Both theuser response and the set of expected parts of speech each correspond tothe same goal-directed auditory prompt. If the syntactical error isdetermined to have occurred, the error recovery module 116 provides asyntactical error prompt in response to the syntactical error. Thesyntactical error recovery prompt, according to another embodiment, canprovide to the user a re-phrased version of the goal-directed prompt towhich the user was responding when the syntactical error occurred. Thisre-phrased version can be based on an alternative syntax, such asproviding the goal-directed prompt in the form of a sentence instead ofa question.

Referring again to the earlier scenario concerning a directoryassistance application, the following dialog provides an example of auser response, deemed by the error recovery module 116 to be asyntactical error, and the alternative syntax error-recovery promptsupplied by the error recovery module in response thereto:

-   -   (System): “Welcome to directory assistance. What is the name of        the person you wish to contact?”    -   (Caller): “I wanted to talk to someone about getting some        information about one of your products.”    -   (System): “You may say the name of a specific person or specific        department, such as Bob Jones or Accounting.”

A conceptual error can occur in at least two different ways. Forexample, a conceptual error can occur whenever the user fails to provideany auditory response within a predefined time interval. Accordingly,the error recovery module 116 determines that a conceptual error hasoccurred if the user fails to provide an auditory response to agoal-directed auditory prompt within the predefined time interval. Ifthe error recovery module 116 determines based upon the absence of atimely user response to a goal-directed prompt that a conceptual errorindeed has occurred, the error recovery module responds by providing aconceptual error prompt.

The following dialog illustrates how the error recovery module 116 mighthandle such a communication error in the context of the directoryassistance application:

-   -   (System): “Welcome to directory assistance. What is the name of        the person you wish to contact?”    -   (Caller): NO RESPONSE (time-out expiration)    -   (System): “This directory response provides contact information        for employees and departments. Or, if you would like to speak to        a customer service representative, please respond by saying        OPERATOR.”

At least one other way in which a conceptual error can occur is if theuser response to a particular goal-directed auditory prompt isinconsistent with the context of the particular prompt. In such event,the error recovery module 116 determines that a conceptual error in theform of an out-of-context user response has occurred. In responsethereto, the error recovery module 116 provides a conceptual errorprompt.

The error recovery effected by the system 104 can further includedetermining a type of the communication error. The type of a particularcommunication error is determined by the error recovery module 116,which can additionally analyze each user response to the sequence of oneor more goal-directed auditory prompts provided by the auditory promptmodule 110. Based upon the user response, the type of the communicationerror is determined. The analysis, more particularly, can comprise asequential analysis in which the error recovery performed by the systemfurther includes determining whether a communication error is a firsttype of communication error corresponding to the hierarchy ofcommunication error types, and, if not, determining whether thecommunication error is a second type corresponding to the hierarchy.This sequential analysis can be performed yet a third or more times asneeded, being repeated until the type of the communication error hasbeen correctly determined from among the hierarchy of communicationerror types.

FIG. 2 provides a flowchart of a method 200 for conducting a dialog viaa user interface according to another embodiment of the presentinvention. As illustrated the method 200 includes at step 202 providingan auditory prompt, which can be part of a sequence comprising one ormore goal-directed auditory prompts. If the user fails to provide aresponse to the auditory prompt within a predetermined time interval, adetermination at step 204 is made that a conceptual error has occurred.Accordingly, a conceptual error clarification message is provided to theuser at step 206.

The method 200 illustratively includes, at step 208, determining whetherthe volume at which the user provides a response is within a desiredaudible range. If not, a determination is made that an acoustic errorhas occurred. In response to such determination, an acoustic errorprompt in the form of a clarification (e.g., instruction to the user tospeak louder or more softly) message is provided in response to anacoustic error at step 210. If the user is unable to discern an auditoryprompt, an error determination based upon an indeterminate grammar ismade at step 212. The auditory prompt is repeated, optionally with apredetermined emphasis of certain words, at step 214 in the event ofsuch an error.

Illustratively, the method 200 further includes determining whether alexical error has occurred at step 216. If so, a lexical error prompt inthe response of a clarification message is provided at step 218. Adetermination of whether a syntactical error has occurred is made atstep 220, and if so, a syntactical error prompt is provided in the forma clarification message at step 222. A determination is alsoillustratively made at step 224 as to whether an out-of-context responseto the goal-directed auditory prompt has been provided by the user, thedetermination being based on whether the response is within an activatedgrammar corresponding to the particular sequence of goal-directedauditory prompts. If an out-of-context error has occurred, a conceptualerror prompt is provided at step 226. At each step, if an error hasoccurred, each of the preceding steps is repeated following provision ofthe particular error recovery prompt that is provided depending on thenature of the communication error.

It should be noted, that this iterative series of determinationsaccording to the method 200 provides a mechanism for determining, atleast approximately, the underlying nature of any communication errorthat occurs. Thus, by way of sequentially analyzing the user response tothe goal-directed auditory prompt, the type of the communication errorcan be determined. The error-recovery prompt that is provided to theuser corresponds to the type of communication error determined to haveoccurred. Accordingly, each error recovery prompt can be targeted touniquely address each communication error in accordance with the type ofthe communication error that has occurred. This process can be repeateduntil the communication error is corrected by the user provided anacceptable response to the goal-directed auditory prompt that initiallygave rise to the communication error. Once each of the steps has beencompleted without an error determination, the next goal-directed promptis provided if any remain in the particular dialog being conducted.

As noted already, the present invention can be realized in hardware,software, or a combination of hardware and software. The presentinvention can be realized in a centralized fashion in one computersystem, or in a distributed fashion where different elements are spreadacross several interconnected computer systems. Any kind of computersystem or other apparatus adapted for carrying out the methods describedherein is suited. A typical combination of hardware and software can bea general purpose computer system with a computer program that, whenbeing loaded and executed, controls the computer system such that itcarries out the methods described herein.

The present invention also can be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

This invention can be embodied in other forms without departing from thespirit or essential attributes thereof. Accordingly, reference should bemade to the following claims, rather than to the foregoingspecification, as indicating the scope of the invention.

1. A method for conducting a user dialog via a speech-based userinterface, the method comprising: providing a sequence of goal-directedauditory prompts, the sequence of goal-directed auditory prompts basedupon a pre-determined user-oriented task; and if a communication erroroccurs, responding to the communication error by initiating an errorrecovery based upon a user response to the sequence of goal-directedauditory prompts; wherein the error recovery comprises providing atleast one error-recovery prompt selected from a hierarchy oferror-recovery prompts, the selection based upon a hierarchy ofcommunication error types.
 2. The method of claim 1, wherein thehierarchy of communication error types comprises at least one of anacoustical error, a lexical error, a syntactical error, and a conceptualerror.
 3. The method of claim 1, wherein the error recovery comprisesdetermining a type of the communication error based upon a user responseto one of the sequence of goal-directed auditory prompts.
 4. The methodof claim 1, wherein the error recovery comprises determining a type ofthe communication error based upon a sequential analysis of a userresponse to one of the sequence of goal-directed auditory prompts. 5.The method of claim 1, wherein the error recovery comprises: determiningwhether an acoustical error has occurred based upon an auditory levelassociated with the user response to at least one of the goal-directedauditory prompts; and if the acoustical error has occurred, providing anacoustical error prompt in response thereto.
 6. The method of claim 1,wherein the error recovery comprises: determining whether a lexicalerror has occurred based upon a search for a match between a userresponse and a list of pre-designated acceptable responses, both theuser response and the list of pre-designated acceptable responsescorresponding to a same one of the goal-directed auditory prompts; andif the lexical error has occurred, providing a lexical error prompt inresponse thereto.
 7. The method of claim 1, wherein the error recoverycomprises: determining whether a syntactical error has occurred basedupon a parts-of-speech evaluation of the user response and a search fora match between parts of speech identified by the parts-of-speechevaluation and parts of speech contained in a set of expected parts ofspeech, both the user response and the set of expected parts of speechcorresponding to a same one of the goal-directed auditory prompts; andif the syntactical error has occurred, providing a syntactical errorprompt in response thereto
 8. The method of claim 1, wherein the errorrecovery comprises: determining whether a conceptual error has occurredbased upon a non-auditory user response within a predefined time period;and if the conceptual error has occurred, providing a conceptual errorprompt in response thereto.
 9. The method of claim 1, wherein the errorrecovery comprises: determining whether a conceptual error has occurredbased upon an out-of-context user response; and if the conceptual errorhas occurred, providing a conceptual error prompt in response thereto.10. A system for conducting a user dialog via a speech-based userinterface, the method comprising: an auditory prompt module forgenerating a sequence of goal-directed auditory prompts, the sequence ofgoal-directed auditory prompts based upon a pre-determined user-orientedtask; and an error recovery module for responding to a communicationerror by initiating an error recovery based upon a user response to thesequence of goal-directed auditory prompts; wherein the error recoverycomprises providing at least one error-recovery prompt sequentiallyselected from a hierarchy of error-recovery prompts, the sequentialselection based upon a hierarchy of communication error types.
 11. Thesystem of claim 10, wherein the hierarchy of communication error typescomprises at least one of an acoustical error, a lexical error, asyntactical error, and a conceptual error.
 12. The system of claim 10,wherein the error recovery comprises determining a type of thecommunication error based upon a user response to one of the sequence ofgoal-directed auditory prompts.
 13. The system of claim 10, wherein theerror recovery comprises determining a type of the communication errorbased upon a sequential analysis of a user response to one of thesequence of goal-directed auditory prompts.
 14. The system of claim 10,wherein the error recovery comprises: determining whether an acousticalerror has occurred based upon an auditory level associated with the userresponse to at least one of the goal-directed auditory prompts; and ifthe acoustical error has occurred, providing an acoustical error promptin response thereto.
 15. The system of claim 10, wherein the errorrecovery comprises: determining whether a lexical error has occurredbased upon a search for a match between a user response and a list ofpre-designated acceptable responses, both the user response and the listof pre-designated acceptable responses corresponding to a same one ofthe goal-directed auditory prompts; and if the lexical error hasoccurred, providing a lexical error prompt in response thereto.
 16. Thesystem of claim 10, wherein the error recovery comprises: determiningwhether a syntactical error has occurred based upon a parts-of-speechevaluation of the user response and a search for a match between partsof speech identified by the parts-of-speech evaluation and parts ofspeech contained in a set of expected parts of speech, both the userresponse and the set of expected parts of speech corresponding to a sameone of the goal-directed auditory prompts; and if the syntactical errorhas occurred, providing a syntactical error prompt in response thereto17. The system of claim 10, wherein the error recovery comprises:determining whether a conceptual error has occurred based upon anon-auditory user response within a predefined time period; and if theconceptual error has occurred, providing a conceptual error prompt inresponse thereto.
 18. The system of claim 10, wherein the error recoverycomprises: determining whether a conceptual error has occurred basedupon an out-of-context user response; and if the conceptual error hasoccurred, providing a conceptual error prompt in response thereto.
 19. Acomputer-readable storage medium for conducting a user dialog via aspeech-based user interface, the storage medium comprising computerinstructions for: providing a sequence of goal-directed auditoryprompts, the sequence of goal-directed auditory prompts based upon apre-determined user-oriented task; and if a communication error occurs,responding to the communication error by initiating an error recoverybased upon a user response to the sequence of goal-directed auditoryprompts; wherein the error recovery comprises providing at least oneerror-recovery prompt selected from a hierarchy of error-recoveryprompts based upon a hierarchy of communication error types.
 20. Thecomputer-readable storage medium of claim 19, wherein the error recoveryfurther comprises determining a type of the communication error basedupon a sequential analysis of a user response to one of the sequence ofgoal-directed auditory prompts.